Surface hardening by heat treatment or nitriding and the like, or surface treatment by shot-peening and the like, are performed to improve the wear-resistance and fatigue strength of steel manufactured products such as gears, shafts, and springs.
Conventionally the evaluation of surface properties such as residual stress and hardness following surface processing of such manufactured products has been performed by sample destructive inspection. This led to problems such as the inability to inspect all manufactured directly, and rendering unusable the inspected, since inspection was destructive.
A strong requirement has therefore emerged to develop a device capable of non-destructively inspecting manufactured product surface properties. As an example of such a device, Patent Document 1 discloses a non-destructive inspection device for a shot-peened surface, wherein an AC (alternating current) signal is input while varying the frequency in an inspection circuit comprising a coil disposed over a shot-peened surface, and the state of residual stress in the inspected object is inspected using impedance frequency response characteristics in the inspection circuit.